On-chip spectroscopic sensors with optical fringe suppression

ABSTRACT

An on-chip spectroscopic sensor includes a tunable diode laser. A laser driver for drives the tunable diode laser. An analyte test cavity receives a chemical sample and exposes the received chemical sample to light from the tunable diode laser. An optical detector detects light emerging from the analyte test cavity as a result of the laser exposure. A spectral analyzer determines a spectrum of the emerging light, matches and removes one or more known optical fringe patterns from the determined spectrum, and determines a composition or concentration of the chemical sample from the optical fringe pattern-removed spectrum.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under DE-AR0000540awarded by the Department of Energy. The government has certain rightsin the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on provisional application Ser. No.62/506,376, filed May 15, 2017, the entire contents of which are hereinincorporated by reference.

BACKGROUND

The present invention relates to spectroscopic sensors and, morespecifically, to on-chip spectroscopic sensors with optical fringesuppression.

Natural gas is an important part of global energy generation. However,as natural gas is collected from wells and carried along pipes, there isa risk that methane may leak. Spectroscopic sensors may therefore beused to detect the presence of fugitive methane. While traditionalspectroscopic sensors may be large, expensive and delicate, on-chipspectroscopic sensors have been developed.

On-chip spectroscopic sensors may utilize laser absorption spectroscopy(LAS) to determine an atomic and/or molecular composition of matter byanalyzing the frequency properties of laser light passing through ananalyte. However, as the laser light of various frequencies passesthrough the on-chip spectroscopic sensors, reflections of light fromunwanted interfaces may lead to various standing wave interferencepatterns, which, if not properly accounted for, may lead to imprecisespectroscopy as the signal may appear to include oscillations. Thiseffect may be known as optical fringing, or etaloning, as it is causedby the light circulating within an optical cavity, which is an etalon.The use of silicon photonic waveguides within the on-chip spectroscopicsensors may be particularly prone to optical fringes with difficult topredict patterns, owing to its relatively large thermo-opticcoefficient.

SUMMARY

An on-chip spectroscopic sensor includes a tunable diode laser. A laserdriver for drives the tunable diode laser. An analyte test cavityreceives a chemical sample and exposes the received chemical sample tolight from the tunable diode laser. An optical detector detects lightemerging from the analyte test cavity as a result of the laser exposure.A spectral analyzer determines a spectrum of the emerging light, matchesand removes one or more known optical fringe patterns from thedetermined spectrum, and determines a composition or concentration ofthe chemical sample from the optical fringe pattern-removed spectrum.

A method for performing spectroscopy includes using a spectroscopicsensor to acquire spectra over a temperature range. A plurality ofoptical fringe patterns are identified at regular temperature incrementswithin the temperature range. The plurality of optical fringe patternsare stored within a database. The spectroscopic sensor is configured toremove background from subsequent acquisitions based on the database ofoptical fringe patterns.

A method for detecting fugitive methane gas includes installing aspectroscopic sensor apparatus into a calibration tool. Thespectroscopic sensor apparatus within the calibration tool is used toidentify a plurality of optical fringe patterns at regular temperatureincrements within a predetermined temperature range. The plurality ofoptical fringe patterns are stored within a database. The spectroscopicsensor apparatus is configured by constructing a background removalmodel that is configured to detect one or more of the plurality ofoptical fringe patterns while correcting for laser frequency rampinginstabilities by allowing each of the plurality of optical fringepatterns to be stretched/contracted. The configured spectroscopic sensorapparatus is removed from the calibration tool. The configuredspectroscopic sensor apparatus is installed to a final location. Thespectroscopic sensor apparatus is installed at the final location todetect fugitive methane.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant aspects thereof will be readily obtained as the same becomesbetter understood by reference to the following detailed descriptionwhen considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram illustrating a laser absorptionspectroscopy (LAS) sensor in accordance with exemplary embodiments ofthe present invention;

FIG. 2 is a schematic diagram illustrating a test and calibration toolfor LAS sensor arrangements in accordance with exemplary embodiments ofthe present invention;

FIG. 3 is a flow chart illustrating an approach for performing fringebackground removal calibration for an LAS in accordance with exemplaryembodiments of the present invention;

FIG. 4 is a flow chart illustrating an approach for performing fringebackground removal with a calibrated LAS in accordance with exemplaryembodiments of the present invention;

FIG. 5 is a flow chart illustrating another approach for performingfringe background removal with a calibrated LAS in accordance withexemplary embodiments of the present invention;

FIG. 6 is a flow chart illustrating a method for using a calibrated LASsensor arrangement to detect fugitive methane gas in accordance withexemplary embodiments of the present invention; and

FIG. 7 shows an example of a computer system capable of implementing themethod and apparatus according to embodiments of the present disclosure.

DETAILED DESCRIPTION

In describing exemplary embodiments of the present invention illustratedin the drawings, specific terminology is employed for sake of clarity.However, the present invention is not intended to be limited to theillustrations or any specific terminology, and it is to be understoodthat each element includes all equivalents.

Exemplary embodiments of the present invention relate to an on-chipspectroscopic sensor having optical fringe suppression for removing theeffects of fringes such as those caused by etalons. However, theapparatus and techniques discussed herein may be applied more generallyto other spectroscopic sensors such as those that are non-integrated andoperate in free-space.

As discussed above, various components of an on-chip spectroscopicsensor may be particularly affected by change in temperature. Forexample, silicon photonic waveguides having relatively largethermo-optic coefficient may be used. Accordingly, the nature of opticalfringes may have a tendency to change as the temperature of the on-chipspectroscopic sensor changes. Exemplary embodiments of the presentinvention may therefor use a testing/calibrating apparatus to observethe effects of optical fringes for a particular on-chip spectroscopicsensor over a wide range of temperatures so that these effects may beidentified, with respect to temperature. These tests may be performedwithout the presence of an analyte gas so as to observe baseline opticalfringing. The on-chip spectroscopic sensor may thereafter be calibratedto identify and remove optical fringes that may be identified based onthe testing/calibration data for that particular device. Additionally,by identifying the particular optical fringe pattern present, exemplaryembodiments of the present invention may determine a temperature in useby determining which fringe pattern has been identified and/or removed.

It is to be understood, however, that even though exemplary embodimentsof the present invention are discussed herein with respect toidentifying and removing optical fringe patterns that are understoodwith respect to temperature, other operating conditions may be used tocharacterize optical fringe patterns, either instead of or in additionto temperature. Examples of other operating conditions may includepressure, humidity, etc.

FIG. 1 is a schematic diagram illustrating a laser absorptionspectroscopy (LAS) sensor in accordance with exemplary embodiments ofthe present invention. While a simplified schematic is provided, it isto be understood that the LAS sensor may be embodied as an integratedon-chip silicon tunable diode laser absorption spectroscopy (IOS-TDLAS)apparatus, as a pigtailed waveguide sensor or as another suitable LASsensor. The LAS sensor arrangement 101 may include a laser driver and atunable diode laser driven therewith 102. The laser driver may beconfigured to drive the diode laser across a desired frequency spectrum.The laser light generated from the laser driver and a tunable diodelaser driven therewith 102 may be directed to an analyte test cavity103, which may be an exposed region in which ambient gas is permitted toenter.

In the analyte test cavity 103, the laser light may collide with gasmolecules thereby exciting the gas molecules, which may then releasephotons of a particular wavelength as they relax. As the wavelength ofthe photons released may be characteristic for the gas within theanalyte test cavity 103, an optical detector 104 may be used to sensethe light emerging from the analyte test cavity 104 and an acquisitioncard 105 may be used to sample the optical detector 104 output, andtransform the sampled signal into the frequency domain. A spectralanalyzer 106 may then be used to identify the nature of the gas withinthe analyte test cavity 103 based upon the spectra observed, forexample, to detect a presence of methane gas.

It is to be understood that various optical waveguides and other opticalelements are included within the LAS sensor arrangement, for example, toguide light from the diode laser to the analyte test cavity, to guidelight from the analyte test cavity to the optical detector, etc.

In practice, the above described detection approach may be complicatedby the presence of background noise, such as the optical fringe patternsthat are formed as light is reflected back and forth along the variousoptical waveguides and other optical elements. These fringe patternsmay, at times, be difficult to isolate from the spectra resulting fromthe relaxation of the gas molecules, particularly as the opticalcharacteristics of the particular LAS sensor arrangement provide uniquefringe patterns. Moreover, the fringe patterns may have a tendency tochange with ambient temperature, and various other ambient conditions.

Exemplary embodiments of the present invention therefore individuallycalibrate LAS sensor arrangements to ascertain the appearance of opticalfringe patterns that are particular to the given LAS sensor arrangementunder test. As these optical fringe patterns have a tendency to changewith respect to temperature and various other ambient conditions,exemplary embodiments of the present invention construct a database ormodel the optical fringe patterns of the particular LAS sensorarrangement across a temperature range that the LAS sensor arrangementis likely to operate under.

FIG. 2 is a schematic diagram illustrating a test and calibration toolfor LAS sensor arrangements in accordance with exemplary embodiments ofthe present invention. The test and calibration tool 20 may include aregion for receiving the LAS sensor arrangement 101 to be tested. Thetool 20 may also include a thermistor 21 for monitoring ambienttemperature, although the inclusion of the thermistor 21 may be omitted,as is described in greater detail below. One or more additional sensors22 may also optionally be used for monitoring other ambient conditionssuch as pressure and humidity. During the testing and calibrationprocess, the LAS sensor arrangement 101 is operational. An analyte gasmay be substantially absent from the LAS sensor arrangement 101 or aknown gas may be used. The resulting spectra acquired by the LAS sensorarrangement 101 may be provided to a processor 23 which may remove aknown spectrum for the known gas, if one is used, and may store theresults in a database 24 by temperature, as recorded by the thermistor21, and by the other ambient conditions as measured by the additionalsensors 22, if they are used. Alternatively, the results may be storedin the database 24 without being related to particular temperatureand/or data from additional sensors 22.

Regardless of whether results are to be related to particulartemperatures, testing may be performed for a period of time long enoughto capture data across an entire temperature range that the LAS sensorarrangement 101 is likely to be operated within, however, aheating/cooling unit 25 may optionally be used to help span the desiredtemperature range.

The processor 23 may additionally use the data acquired from the LASsensor arrangement 101, the thermistor 21, and the additional ambientcondition sensors 22 (if used) to isolate the optical fringe patterns ateach temperature and program (configure) the LAS sensor arrangement 101to identify and remove the identified optical fringe patterns from datathat is subsequently acquired during actual use (use that is not relatedto testing and configuration, for example, use in fugitive methanedetection).

FIG. 3 is a flow chart illustrating an approach for performing fringebackground removal calibration for an LAS in accordance with exemplaryembodiments of the present invention. The LAS sensor arrangement to beconfigured may be installed within a test and calibration tool (StepS301) such as the tool described above with respect to FIG. 2.Thereafter, the LAS sensor arrangement may be tested across a desiredrange of temperatures (Step S302). For example, testing may be performedfrom between 20° C. and 30° C. at intervals of 1×10⁻⁴° C. However, it isto be understood that other temperature ranges may be used and otherintervals may be used. The LAS sensor arrangement may be able to spanthe desired temperature range during the course of normal operation,however, a heating and/or cooling device may optionally be used toensure that the full range of temperatures are achieved during testingand calibration.

The result of this calibration may be stored within a spectral database(Step S302). As mentioned above, while the spectra may be acquired overa particular temperature at regular temperature intervals, the resultsneed not be correlated within the database according to temperature.However, as an optional step, the optical fringe patterns may becorrelated, within the database, according to the particular temperatureat which the particular optical fringe pattern was acquired (Step S303),and this may be accomplished, for example, by measuring temperature asspectra are observed, for example, using the thermistor 21.

It is noted that as the diode laser is tuned across the desiredfrequency range for performing LAS, laser ramping nonlinearities mayimpact the appearance of the optical fringe patterns and may make itmore difficult to match the optical fringe patterns observed duringcalibration with those that might present during actual use.Accordingly, exemplary embodiments of the present invention may furtherallow correction of laser frequency ramping instabilities by allowingthe measured fringe pattern to be stretched/contracted in order tobetter approximate the optical fringe patterns stored in the database.Once properly matched to the fringe database (linear or splineinterpolation may be employed to stretch/contract the fringe pattern), apre-determined frequency axis (measured in Step S304) can be reliablyused for concentration retrieval.

The LAS sensor arrangement may then be calibrated according to thedatabase or mode (Step S305). This calibration step may include copyingthe database or model to local storage of the LAS sensor arrangement andconfiguring the LAS sensor arrangement to search for an identify thepresence of one of the stored optical fringe patterns during use. TheLAS sensor arrangement may be configured to remove optical fringepatterns, that have been identified, from sensor data so as to producesensor data that is substantially free of optical fringe patterns. Onceconfiguration has been completed, the LAS sensor arrangement may beremoved from the tool and integrated into a final form, such as a toolfor detecting fugitive methane (Step S306).

FIG. 4 is a flow chart illustrating an approach for performing fringebackground removal with a calibrated LAS in accordance with exemplaryembodiments of the present invention. The LAS sensor arrangement, havingbeen calibrated, for example, according to the approach described abovewith reference to FIG. 3, may be used to obtain sensor data (Step S401).This may include, for example, tuning the diode laser across a desiredfrequency range, obtaining optical detector signal, and digitizing theoptical detector signal. From there, the database of known opticalfringe patterns may be drawn upon to identify an optical fringe patternfrom the sensor data. The optical fringe patterns of the database, whenclosely matched to optical fringe patterns found in the obtained sensordata, represent background that is to be removed. Each optical fringepattern in the database may be compared, for example, one-by-one, to thenewly acquired spectrum (Step S402).

The correct database fringe pattern for the sensor data spectrum isdetermined by constructing a fitting model that allowsleast-mean-squares (LMS) fitting to be performed (Step S403).

The fitting model may be constructed, for example, using the followingformula:I _(t)(v(t))=I ₀(v(t))×e ^(−σ(v(t))NL)×fringe backgroundwhere v(t) represents laser frequency as a function of time. In directabsorption spectroscopy, the laser frequency may be linearly tunedacross the absorption line periodically. I_(t) represents transmittedlaser intensity or equivalently the measured spectrum. I₀ representsincident laser intensity, which for example can be modeled as alow-order polynomial to approximate changes in laser output intensity aslaser frequency is being tuned. σ represents cross section of lightabsorption by a single particle; and this value may be modeled using theabsorption lineshape function and HITRAN absorption line parameters. Lrepresents optical pathlength of the sensor. N represents analyteconcentration. The fringe background represents a fringe patternspectrum from the database. The fringe background may be configured toallow for the adjustment of laser frequency axis v(t) during fitting toaccommodate stretching and contraction of the fringe pattern, and can beimplemented using linear interpolation of the measured spectrum.

Thereafter, least mean squared (LMS) fitting may be used to apply theconstructed fitting model to the sensor data (Step S404). By performingLMS fitting, the following parameters of the fitting model may bedetermined: the adjusted laser frequency axis v(t), the fringebackground amplitude scaling, the incident laser intensity I₀, and theanalyte concentration N.

As discussed above, the adjustment of the laser frequency axis v(t) maybe used to account for the effects that laser frequency shift may haveon the appearance of the optical fringe patterns. In this way, theoptical frequency axis may be calibrated to account for any laserfrequency fluctuations or drift. The corrected frequency axis may thenbe used in the fitting of the absorption profile, which may ensureaccurate concentration retrieval.

The results of the LMS fitting may then be evaluated to determine ifthere is a match between one of the known optical fringe patterns andthe sensor data (Step S405). This may be performed, for example, bydetermining whether a minimum residual is obtained as a result of theLMS fitting (Step S406). If the minimum residual is not obtained (No,Step S406), then the next optical fringe pattern within the database maybe checked against the sensor data, thereby repeating the process fromStep S402 until a minimum residual is obtained (Yes, Step S406), therebysignifying a match between the optical fringe pattern being tested andthe sensor data. Thereafter, the analyte concentration already retrievedduring LMS fitting (Step S404) may be reported as the fringe-correctedconcentration as the LMS fitting was performed with the best-matchfringe background (Step S407).

FIG. 5 is a flow chart illustrating another approach for performingfringe background removal with a calibrated LAS in accordance withexemplary embodiments of the present invention. According to thisapproach, calibration would have to include the optional step S303discussed above in which optical fringe patterns were correlated withtemperature. During sensing, the sensor data may be obtained (Step S501)and the obtained sensor data may be transformed into the Fourier domain(Step S502). The present temperature may be identified based on anobserved optical fringe pattern within the obtained sensor data in theFourier domain (Step S503). Specifically, the phase of the Fouriertransformed spectrum corresponding to one or more etalon fringes may beused to quantify the amount of etalon drift (which is directly relatedto sensor temperature) in the sensor data compared to a prior time.Then, the optical fringe pattern associated with the determinedtemperature may be retrieved (Step S504) and used to construct a fittingmodel (Step S505) and then the fitting model may be used to perform LMSfitting on the obtained sensor data to produce the background-removedspectrum, which may be analyzed to determine analyte concentration (StepS506).

As mentioned in the approach above, the temperature of the LAS sensormay be determined by seeing which temperature is associated with thematched optical fringe pattern. The determined temperature may then bereported. According to some exemplary embodiments of the presentinvention, determining the temperature in this way may be used as acontinuous on-chip temperature monitor. It is further noted that used assuch, exemplary embodiments of the present invention may be used todetermine temperature to a resolution equal to that used duringcalibration, which may be on the order of 1×10⁻⁴° C., as describedabove, which may be significantly more accurate than other on-chiptemperature measurement devices.

If the temperature is approximately known, for example, by the use of athermistor incorporated into the LAS sensor arrangement, thisapproximate temperature may be used to narrow down the field of thematching so as to limit the search to known optical fringe patternsassociated with temperatures within a particular range around theapproximate temperature. For example, if the temperature is estimated tobe 24° C., matching may be limited to +/−2° C. of the estimatedtemperature, which may be 22° C. to 26° C. By limiting the search range,the procedure described in FIG. 4 may be implemented faster and withreduced computational cost.

According to either approach, the background-removed spectrum may becompared against known spectra, such as the known spectrum for methane,to determine if methane is present. FIG. 6 is a flow chart illustratinga method for using a calibrated LAS sensor arrangement to detectfugitive methane gas in accordance with exemplary embodiments of thepresent invention. First, the LAS sensor may be calibrated, for example,as discussed above (Step S601). Then, the calibrated LAS sensor may beinstalled along a path of methane supply lines or a facility handlingmethane (Step S602). The LAS sensors may be installed at locations wheremethane should not be, but would likely be found in the event of a leak.Sensing may then be continuously performed, using optical fringe patternbackground removal, for example, as described in detail above (StepS603). The sensing may then be used to determine whether fugitivemethane gas is detected (Step S603).

If it is determined that methane is present (Yes, Step S603), then analert may be generated (Step S604). If methane is not determined to bepresent (No, Step S603) then the process may be repeated.

It is also noted that exemplary embodiments of the present invention maybe used to identify and remove multiple different optical fringes from asingle detection signal as different optical fringes may be present atdifferent free spectral ranges (FSR) and it is understood thatacquisition of sensor data (for both calibration and use) may beperformed across a wide set of spectral ranges as the diode laser istuned. Additionally, the presence of multiple optical fringes may beused to determine that different parts of the LAS sensor arrangement mayhave different temperatures, and to identify the different temperatureat each part of the LAS sensor arrangement.

The alert, so generated, may take the form of an audible and/or visualalert such as an alarm. Speakers and lights may be used to implementsuch an alarm. Alternatively, or additionally, the alert may be embodiedas an automatically generated email and/or text message that istransmitted to a mobile device of an operator, for example, over awireless network.

FIG. 7 shows another example of a system in accordance with someembodiments of the present invention. By way of overview, someembodiments of the present invention may be implemented in the form of asoftware application running on one or more (e.g., a “cloud” of)computer system(s), for example, mainframe(s), personal computer(s)(PC), handheld computer(s), client(s), server(s), peer-devices, etc. Thesoftware application may be implemented as computer readable/executableinstructions stored on a computer readable storage media (discussed inmore detail below) that is locally accessible by the computer systemand/or remotely accessible via a hard wired or wireless connection to anetwork, for example, a local area network, or the Internet.

Referring now to FIG. 7, a computer system (referred to generally assystem 1000) may include, for example, a processor e.g., centralprocessing unit (CPU) 1001, memory 1004 such as a random access memory(RAM), a printer interface 1010, a display unit 1011, a local areanetwork (LAN) data transmission controller 1005, which is operablycoupled to a LAN interface 1006 which can be further coupled to a LAN, anetwork controller 1003 that may provide for communication with a PublicSwitched Telephone Network (PSTN), one or more input devices 1009, forexample, a keyboard, mouse etc., and a bus 1002 for operably connectingvarious subsystems/components. As shown, the system 1000 may also beconnected via a link 1007 to a non-volatile data store, for example,hard disk, 1008.

In some embodiments, a software application is stored in memory 1004that when executed by CPU 1001, causes the system to perform acomputer-implemented method in accordance with some embodiments of thepresent invention, e.g., one or more features of the methods, describedwith reference to FIGS. 3 and 4.

The present invention may be a system, a method, and/or a computerprogram product at any possible technical detail level of integration.The computer program product may include a computer readable storagemedium (or media) having computer readable program instructions thereonfor causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, configuration data for integrated circuitry, oreither source code or object code written in any combination of one ormore programming languages, including an object oriented programminglanguage such as Smalltalk, C++, or the like, and procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The computer readable program instructions may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider). In some embodiments, electronic circuitry including,for example, programmable logic circuitry, field-programmable gatearrays (FPGA), or programmable logic arrays (PLA) may execute thecomputer readable program instructions by utilizing state information ofthe computer readable program instructions to personalize the electroniccircuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the Figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Exemplary embodiments described herein are illustrative, and manyvariations can be introduced without departing from the spirit of theinvention or from the scope of the appended claims. For example,elements and/or features of different exemplary embodiments may becombined with each other and/or substituted for each other within thescope of this invention and appended claims.

What is claimed is:
 1. A method for detecting fugitive methane gas,comprising: installing a spectroscopic sensor apparatus into acalibration tool; using the spectroscopic sensor apparatus within thecalibration tool to identify a plurality of optical fringe patterns atregular temperature increments within a predetermined temperature range;store the plurality of optical fringe patterns within a database;configure the spectroscopic sensor apparatus by constructing abackground removal model that is configured to detect one or more of theplurality of optical fringe patterns while correcting for laserfrequency ramping instabilities by allowing each of the plurality ofoptical fringe patterns to be stretched/contracted; removing theconfigured spectroscopic sensor apparatus from the calibration tool;installing the configured spectroscopic sensor apparatus to a finallocation; and using the spectroscopic sensor apparatus installed at thefinal location to detect fugitive methane, wherein using thespectroscopic sensor apparatus to detect fugitive methane includes:acquiring a subsequent spectrum using the spectroscopic sensorapparatus; removing background from the acquired subsequent spectrumusing the constructed background removal model; and identifying thepresence of fugitive methane by analyzing the background-removedspectrum, and wherein the background removal model is constructed usingthe formula:I _(t)(v(t))=I ₀(v(t))×e ^(−σ(v(t))NL)×fringe background where v(t)represents laser frequency as a function of time, I_(t) presentstransmitted laser intensity or equivalently the measured spectrum, I₀represents incident laser intensity, σ represents cross section of lightabsorption by a single particle, L represents optical pathlength of thesensor, and N represents analyte concentration.
 2. The method of claim1, wherein the predetermined temperature range is from 20° C. to 30° C.and the regular temperature intervals are 1×10⁻⁴⁰ C.
 3. The method ofclaim 1, wherein the plurality of optical fringe patterns are storedwithin the database in correspondence with a temperature of acquisitionand in the background removal model is further used to determine acurrent temperature by determining a temperature that corresponds to amatched optical fringe pattern of the plurality of optical fringepatterns.
 4. The method of claim 1, wherein configure the spectroscopicsensor apparatus further includes copying the database to thespectroscopic sensor apparatus.
 5. The method of claim 1, wherein thebackground removal model allows for least-mean-squares (LMS) fitting. 6.The method of claim 1, wherein each of the plurality of optical fringepatterns is allowed to be stretched/contracted by adjusting the laserfrequency axis v(t).
 7. The method of claim 1, wherein the finallocation is a methane conduit or facility handling methane.